In the dental clinical field, a dental curing composition also called dental filling composite resin is used to esthetically and functionally restore a lost portion of a tooth caused by dental caries, tooth fracturing, etc. The dental curing composition is generally prepared by mixing a matrix resin including several types of polymerizable monomers, various filling materials such as an inorganic filler, an organic-inorganic composite filler, etc., and a polymerization catalyst with each other into a paste form.
Requirements demanded to a dental curing composition can be an excellent handling property for a dentist to execute a filling operation in addition to a high mechanical property, and a color tone and light permeability similar to those of a natural tooth. Describing the handling property in detail, it is preferable that a paste does not stick to a filling instrument when the dental curing composition is injected to fill a cavity or when shaping is executed to restore a tooth form. Further, a character as a stretchy paste tends to be preferred for restoring an anterior tooth while a character as a properly packable paste tends to be preferred for restoring a posterior tooth. The dental curing composition shrinks during polymerization curing and a stress is therefore applied to an adhesive interface between the dental curing composition and the tooth substance caused by shrinkage, and a gap is formed. Thereby, a risk arises that a postoperative pain is caused. Especially, for a posterior tooth, the risk is increased because a high bite pressure is applied thereto. Based on this, a low polymerization shrinkage property is also an important requirement demanded to a dental curing composition.
An approach to cause a packable paste character preferred for restoring a posterior tooth to develop can be an approach of filling a matrix resin with inorganic filler having an average particle diameter of about 0.1 to 5 μm at a high ratio. However, a high-ratio filling is difficult because inorganic filler having such a particle diameter has a large surface area. As a result, the content of the inorganic filler is small and the ratio of the matrix resin is high and shrinkage therefore becomes significant during polymerization curing of the dental curing composition.
On the other hand, Japanese Laid-Open Patent publication No. 2011-98941 proposes a technique according to which hardening of the paste character over time is suppressed by impregnating in advance an organic-inorganic composite filler with a polymerizable monomer whose viscosity at 60° C. is equal to or lower than 10 Pa·s. With this technique, however, a problem arises that the production of the organic-inorganic composite filler becomes complicated because an impregnation process for the polymerizable monomer is added.
Organic-inorganic composite filler having an average particle diameter of about several μm to 30 μm is used in addition to inorganic filler, as a filling material blended in the dental curing composition. The organic-inorganic composite filler is produced by mixing and polymerizing an inorganic filler of about several nm to 0.5 μm with a matrix resin and, thereafter, crushing the polymerized mixture. Generally, organic-inorganic composite filler is not blended alone in a dental curing composition but is blended therein together with the above inorganic filler (Japanese Patent publication No. 7-91170 and Japanese Patent No. 3276388). The organic-inorganic composite filler has advantages that the organic-inorganic composite filler can reduce stickiness of the paste and that the organic-inorganic composite filler can fill the matrix resin at a high ratio.
However, the paste character of the dental curing composition including the organic-inorganic composite filler blended therein tends to be hardened over time due to adsorption of the polymerizable monomer in the matrix resin by the organic substance portion of the organic-inorganic composite filler. Especially, when the amount of filled filler is large, this tendency becomes more noticeable. This phenomenon can be suppressed to some extent by softening the paste character in advance by reducing the amount of filled filler. With this countermeasure, however, no packable paste character can be acquired that is excellent in the shaping property suitable for restoring a posterior tooth, and a problem also arises such as that the polymerization shrinkage becomes significant.